The test, which took place Sept. 3, was the country’s sixth and largest underground nuclear test, according to Reuters.

“They’re probably relaxation events from the sixth nuclear test. When you have a large nuclear test, it moves the earth’s crust around the area, and it takes a while for it to fully subside. We’ve had a few of them since the sixth nuclear test,” a U.S. Geological Survey official told Reuters.

There have been a number of tremors felt in the area since the test, prompting experts to think that the mountainous area may have been damaged as a result of the tests, Reuters reported.

Pyongyang claimed that they tested a hydrogen bomb, and has suggested that its next test would be conducted above ground. North Korean leader Kim Jong-un has also said that all of the United States is now within the country’s range.